Corrosion is an electrochemical process that takes place at the surface of a metal substrate when exposed to water, oxygen, and other corrosive elements causing the metal to deteriorate. Coating a metal substrate creates a barrier that helps reduce exposure to these deleterious elements, thus reducing the corrosion of the metal. However, if there is a defect or imperfection in the coating film, coating failure becomes imminent and corrosion ensues.
For many years, chromate-based chemistries have been the benchmark of corrosion inhibitors incorporated into coatings for providing corrosion protection for metal substrates. Due to the inherent toxicity of and strict environmental regulations concerning chromate-based corrosion inhibitors, paint manufacturers have been forced to replace these toxic chromate-based inhibitors with more benign alternatives. Although several non-toxic inhibitors are commercially available, including inorganic borates, phosphates, phosphites, ion exchange silicas, ferrites, titanates, vanadates, molybdates, zirconates, and organic inhibitors such as amines, phosphonates, and sulfonates, these inhibitors fail to meet the outstanding performance exhibited by chromate-based corrosion inhibitors.
Chromate-based corrosion inhibitors are particularly effective in thin film applications where the barrier effect of the coating, the protection provided by an impermeable film from a corrosive environment, is virtually non-existent. Corrosion protection must, therefore be provided by corrosion inhibitors in thin film applications. In thin film coatings (generally defined as those coatings between approximately 1-5 mils thick), for example, those used in coils, in aerospace and in powder coatings, corrosion protection is provided by the inhibitive action of a single corrosion inhibitor alone. Non-toxic inhibitors provide corrosion protection through anodic, cathodic, film formation, adsorption or precipitation mechanisms. Most non-toxic inhibitors utilize a single type of corrosion inhibiting mechanism. As a result, these non-toxic corrosion inhibitors fail to provide the same corrosion resistance as their toxic counterparts.
Film formation is a mechanism involving molecules whose structure contains a polar head-group and a non-polar tail group. The film is formed when the adhering head-group elements, for example, nitrogen, sulfur, silicon, or phosphorus, chemically bind to a polar metal surface while the tail group remains oriented away into the non-polar coating, thus providing an impermeable layer that restricts corrosive ions from reaching the substrate.
Chromate-based inhibitors are known for their great performance primarily because they act as mixed corrosion inhibitors, i.e., anodic and cathodic inhibitors. All corrosion reactions have an anodic and cathodic component. Anodic and cathodic corrosion inhibitors inhibit corrosion by shifting the electrochemical reaction potential of the substrate either more positive (anodic) or negative (cathodic). Thus, chromates can (a) prevent corrosion by passivating the substrate and shifting the substrate potential more positive (anodic), thus reducing the corrosion rate of the substrate and (b) by depositing insoluble hydroxide films, which shift the substrate potential more negative (cathodic), thus reducing the severity of metal corrosion.
An anodic inhibitor prevents the metal from going into solution as a metal cation as shown in Reaction 1.Reaction 1: M(s)→Mn+(aq)+ne−Where M(s) is the metal, Mn+ is the metal ion going into the aqueous solution, and ne− is the number of electrons liberated in the process.
A stable product is formed at the surface, which prevents further dissolution. A cathodic inhibitor deposits a protective film on the substrate, which in turn prevents the reduction of oxygen (Reaction 2) and hydrogen (Reaction 3) at the surface.Reaction 2: O2+2H2O+4e−→4OH−  (2)Reaction 3: 2H++2e−→H2  (3)
A cathodic inhibitor essentially shuts down the opposite reaction to the anodic process.
Unlike chromate-based corrosion inhibitors, most non-toxic inhibitors require solubility in the organic coating in order to be effective. Permeating water or moisture in the coating hydrolyzes these inhibitive non-toxic salt inhibitors (e.g. barium metaborate) in order to release the non-toxic inhibitor (e.g. borate ions), which provides the corrosion protection. Often, the higher solubility of non-toxic inhibitors compared to chromates contributes to problems such as osmotic blistering of the coating because soluble salts accumulate in the coating and promote the accumulation of moisture into the film, facilitating corrosion. Thus, osmotic blistering is a common problem associated with non-toxic corrosion inhibitors having higher solubility levels (e.g. borates) because such inhibitors may leach or dissolve too quickly into the coating, become too quickly consumed, and thus afford no long-term corrosion protection properties to the coating. The inhibitor of the present invention is only slightly soluble, and therefore significantly reduces the likelihood of osmotic blistering and provides long-term corrosion protection.
Also unlike chromate-based inhibitors, which are more ubiquitous, non-toxic inhibitors are generally more resin and substrate specific. They work well in some resin systems but not in others, varying with the resultant solubility. Chromate-based corrosion inhibitors are to a lesser extent substrate or resin specific and are very effective on ferrous and non-ferrous substrates. Consequently, while many of the commercially available non-toxic inhibitors provide some level of corrosion inhibition there is a definite need for non-toxic corrosion inhibitors having improved performance in order to achieve a similar level of performance as chromate-based corrosion inhibitors.
The present invention fulfills this need, and overcomes the disadvantages and/or shortcomings of known prior art metal substrate corrosion inhibitors and provides a significant improvement thereover. Thus, the present invention provides an improved non-toxic corrosion inhibitor by utilizing multiple inhibitive mechanisms. Unlike the present invention, none of the known commercially available non-toxic inhibitors teach synergy as a means to produce more effective corrosion inhibitors.